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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="KERNEL-RESOURCES"
>17.4. Managing Kernel Resources</A
></H1
><P
>   A large <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> installation can quickly exhaust
   various operating system resource limits. (On some systems, the
   factory defaults are so low that you don't even need a really
   <SPAN
CLASS="QUOTE"
>"large"</SPAN
> installation.) If you have encountered this kind of
   problem, keep reading.
  </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SYSVIPC"
>17.4.1. Shared Memory and Semaphores</A
></H2
><P
>    Shared memory and semaphores are collectively referred to as
    <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
>
    <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
>"</SPAN
> (together with message queues, which are not
    relevant for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>). Almost all modern
    operating systems provide these features, but many of them don't have
    them turned on or sufficiently sized by default, especially as
    available RAM and the demands of database applications grow.
    (On <SPAN
CLASS="SYSTEMITEM"
>Windows</SPAN
>,
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides its own replacement
    implementation of these facilities, so most of this section
    can be disregarded.)
   </P
><P
>    The complete lack of these facilities is usually manifested by an
    <SPAN
CLASS="ERRORNAME"
>Illegal system call</SPAN
> error upon server start. In
    that case there is no alternative but to reconfigure your
    kernel.  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> won't work without them.
    This situation is rare, however, among modern operating systems.
   </P
><P
>    When <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> exceeds one of the various hard
    <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> limits, the server will refuse to start and
    should leave an instructive error message describing the problem
    and what to do about it. (See also <A
HREF="server-start.html#SERVER-START-FAILURES"
>Section 17.3.1</A
>.) The relevant kernel
    parameters are named consistently across different systems; <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 17-1</A
> gives an overview. The methods to set
    them, however, vary. Suggestions for some platforms are given below.
   </P
><DIV
CLASS="TABLE"
><A
NAME="SYSVIPC-PARAMETERS"
></A
><P
><B
>Table 17-1. <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> Parameters</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Description</TH
><TH
>Reasonable values</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMAX</TT
></TD
><TD
>Maximum size of shared memory segment (bytes)</TD
><TD
>at least several megabytes (see text)</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMIN</TT
></TD
><TD
>Minimum size of shared memory segment (bytes)</TD
><TD
>1</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMALL</TT
></TD
><TD
>Total amount of shared memory available (bytes or pages)</TD
><TD
>if bytes, same as <TT
CLASS="VARNAME"
>SHMMAX</TT
>; if pages, <TT
CLASS="LITERAL"
>ceil(SHMMAX/PAGE_SIZE)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMSEG</TT
></TD
><TD
>Maximum number of shared memory segments per process</TD
><TD
>only 1 segment is needed, but the default is much higher</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMNI</TT
></TD
><TD
>Maximum number of shared memory segments system-wide</TD
><TD
>like <TT
CLASS="VARNAME"
>SHMSEG</TT
> plus room for other applications</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMNI</TT
></TD
><TD
>Maximum number of semaphore identifiers (i.e., sets)</TD
><TD
>at least <TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers + 4) / 16)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMNS</TT
></TD
><TD
>Maximum number of semaphores system-wide</TD
><TD
><TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</TT
> plus room for other applications</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMSL</TT
></TD
><TD
>Maximum number of semaphores per set</TD
><TD
>at least 17</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMAP</TT
></TD
><TD
>Number of entries in semaphore map</TD
><TD
>see text</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMVMX</TT
></TD
><TD
>Maximum value of semaphore</TD
><TD
>at least 1000 (The default is often 32767; do not change unless necessary)</TD
></TR
></TBODY
></TABLE
></DIV
><P
>     The most important
    shared memory parameter is <TT
CLASS="VARNAME"
>SHMMAX</TT
>, the maximum size, in
    bytes, of a shared memory segment. If you get an error message from
    <CODE
CLASS="FUNCTION"
>shmget</CODE
> like <SPAN
CLASS="QUOTE"
>"Invalid argument"</SPAN
>, it is
    likely that this limit has been exceeded.  The size of the required
    shared memory segment varies depending on several
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> configuration parameters, as shown in
    <A
HREF="kernel-resources.html#SHARED-MEMORY-PARAMETERS"
>Table 17-2</A
>.  (Any error message you might
    get will include the exact size of the failed allocation request.)
    You can, as a temporary solution, lower some of those settings to
    avoid the failure.  While it is possible to get
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> to run with <TT
CLASS="VARNAME"
>SHMMAX</TT
> as small as
    2 MB, you need considerably more for acceptable performance.  Desirable
    settings are in the hundreds of megabytes to a few gigabytes.
   </P
><P
>    Some systems also have a limit on the total amount of shared memory in
    the system (<TT
CLASS="VARNAME"
>SHMALL</TT
>).  Make sure this is large enough
    for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> plus any other applications that
    are using shared memory segments.  Note that <TT
CLASS="VARNAME"
>SHMALL</TT
>
    is measured in pages rather than bytes on many systems.
   </P
><P
>    Less likely to cause problems is the minimum size for shared
    memory segments (<TT
CLASS="VARNAME"
>SHMMIN</TT
>), which should be at most
    approximately 500 kB for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (it is
    usually just 1). The maximum number of segments system-wide
    (<TT
CLASS="VARNAME"
>SHMMNI</TT
>) or per-process (<TT
CLASS="VARNAME"
>SHMSEG</TT
>) are unlikely
    to cause a problem unless your system has them set to zero.
   </P
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uses one semaphore per allowed connection
    (<A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
>) and allowed autovacuum worker
    process (<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
>), in sets of 16.
    Each such set will
    also contain a 17th semaphore which contains a <SPAN
CLASS="QUOTE"
>"magic
    number"</SPAN
>, to detect collision with semaphore sets used by
    other applications. The maximum number of semaphores in the system
    is set by <TT
CLASS="VARNAME"
>SEMMNS</TT
>, which consequently must be at least
    as high as <TT
CLASS="VARNAME"
>max_connections</TT
> plus
    <TT
CLASS="VARNAME"
>autovacuum_max_workers</TT
>, plus one extra for each 16
    allowed connections plus workers (see the formula in <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 17-1</A
>).  The parameter <TT
CLASS="VARNAME"
>SEMMNI</TT
>
    determines the limit on the number of semaphore sets that can
    exist on the system at one time.  Hence this parameter must be at
    least <TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers + 4) / 16)</TT
>.
    Lowering the number
    of allowed connections is a temporary workaround for failures,
    which are usually confusingly worded <SPAN
CLASS="QUOTE"
>"No space
    left on device"</SPAN
>, from the function <CODE
CLASS="FUNCTION"
>semget</CODE
>.
   </P
><P
>    In some cases it might also be necessary to increase
    <TT
CLASS="VARNAME"
>SEMMAP</TT
> to be at least on the order of
    <TT
CLASS="VARNAME"
>SEMMNS</TT
>. This parameter defines the size of the semaphore
    resource map, in which each contiguous block of available semaphores
    needs an entry. When a semaphore set is freed it is either added to
    an existing entry that is adjacent to the freed block or it is
    registered under a new map entry. If the map is full, the freed
    semaphores get lost (until reboot). Fragmentation of the semaphore
    space could over time lead to fewer available semaphores than there
    should be.
   </P
><P
>    The <TT
CLASS="VARNAME"
>SEMMSL</TT
> parameter, which determines how many
    semaphores can be in a set, must be at least 17 for
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
   </P
><P
>    Various other settings related to <SPAN
CLASS="QUOTE"
>"semaphore undo"</SPAN
>, such as
    <TT
CLASS="VARNAME"
>SEMMNU</TT
> and <TT
CLASS="VARNAME"
>SEMUME</TT
>, do not affect
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
   </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><SPAN
CLASS="SYSTEMITEM"
>AIX</SPAN
></DT
><DD
><P
>        At least as of version 5.1, it should not be necessary to do
        any special configuration for such parameters as
        <TT
CLASS="VARNAME"
>SHMMAX</TT
>, as it appears this is configured to
        allow all memory to be used as shared memory.  That is the
        sort of configuration commonly used for other databases such
        as <SPAN
CLASS="APPLICATION"
>DB/2</SPAN
>.</P
><P
> It might, however, be necessary to modify the global
       <TT
CLASS="COMMAND"
>ulimit</TT
> information in
       <TT
CLASS="FILENAME"
>/etc/security/limits</TT
>, as the default hard
       limits for file sizes (<TT
CLASS="VARNAME"
>fsize</TT
>) and numbers of
       files (<TT
CLASS="VARNAME"
>nofiles</TT
>) might be too low.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>BSD/OS</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Shared Memory. </B
>         By default, only 4 MB of shared memory is supported. Keep in
         mind that shared memory is not pageable; it is locked in RAM.
         To increase the amount of shared memory supported by your
         system, add something like the following to your kernel configuration
         file:
</P><PRE
CLASS="PROGRAMLISTING"
>options "SHMALL=8192"
options "SHMMAX=\(SHMALL*PAGE_SIZE\)"</PRE
><P>
         <TT
CLASS="VARNAME"
>SHMALL</TT
> is measured in 4 kB pages, so a value of
         1024 represents 4 MB of shared memory. Therefore the above increases
         the maximum shared memory area to 32 MB.
         For those running 4.3 or later, you will probably also need to increase
         <TT
CLASS="VARNAME"
>KERNEL_VIRTUAL_MB</TT
> above the default <TT
CLASS="LITERAL"
>248</TT
>.
         Once all changes have been made, recompile the kernel, and reboot.
        </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Semaphores. </B
>         You will probably want to increase the number of semaphores
         as well; the default system total of 60 will only allow about
         50 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> connections.  Set the
         values you want in your kernel configuration file, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>options "SEMMNI=40"
options "SEMMNS=240"</PRE
><P>
        </P
></DIV
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
></DT
><DD
><P
>        The default settings are only suitable for small installations
        (for example, default <TT
CLASS="VARNAME"
>SHMMAX</TT
> is 32
        MB). Changes can be made via the <TT
CLASS="COMMAND"
>sysctl</TT
> or
        <TT
CLASS="COMMAND"
>loader</TT
> interfaces.  The following
        parameters can be set using <TT
CLASS="COMMAND"
>sysctl</TT
>:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmall=32768</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmmax=134217728</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.semmap=256</KBD
></PRE
><P>
        To have these settings persist over reboots, modify
        <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
       </P
><P
>        The remaining semaphore settings are read-only as far as
        <TT
CLASS="COMMAND"
>sysctl</TT
> is concerned, but can be changed
        before boot using the <TT
CLASS="COMMAND"
>loader</TT
> prompt:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmni=256</KBD
>
<SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmns=512</KBD
>
<SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmnu=256</KBD
></PRE
><P>
        Similarly these can be saved between reboots in
        <TT
CLASS="FILENAME"
>/boot/loader.conf</TT
>.
       </P
><P
>        You might also want to configure your kernel to lock shared
        memory into RAM and prevent it from being paged out to swap.
        This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
        setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
       </P
><P
>        If running in FreeBSD jails by enabling <SPAN
CLASS="APPLICATION"
>sysctl</SPAN
>'s
        <TT
CLASS="LITERAL"
>security.jail.sysvipc_allowed</TT
>, <SPAN
CLASS="APPLICATION"
>postmaster</SPAN
>s
        running in different jails should be run by different operating system
        users.  This improves security because it prevents non-root users
        from interfering with shared memory or semaphores in different jails,
        and it allows the PostgreSQL IPC cleanup code to function properly.
        (In FreeBSD 6.0 and later the IPC cleanup code does not properly detect
        processes in other jails, preventing the running of postmasters on the
        same port in different jails.)
       </P
><P
>        <SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
> versions before 4.0 work like
        <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> (see below).
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
></DT
><DD
><P
>        In <SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
> 5.0 and later,
        IPC parameters can be adjusted using <TT
CLASS="COMMAND"
>sysctl</TT
>,
        for example:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmmax=16777216</KBD
></PRE
><P>
        To have these settings persist over reboots, modify
        <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
       </P
><P
>        You might also want to configure your kernel to lock shared
        memory into RAM and prevent it from being paged out to swap.
        This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
        setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
       </P
><P
>        <SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
> versions before 5.0 work like
        <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> (see below), except that
        parameters should be set with the keyword <TT
CLASS="LITERAL"
>options</TT
> not
        <TT
CLASS="LITERAL"
>option</TT
>.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
></DT
><DD
><P
>        The options <TT
CLASS="VARNAME"
>SYSVSHM</TT
> and <TT
CLASS="VARNAME"
>SYSVSEM</TT
> need
        to be enabled when the kernel is compiled. (They are by
        default.) The maximum size of shared memory is determined by
        the option <TT
CLASS="VARNAME"
>SHMMAXPGS</TT
> (in pages). The following
        shows an example of how to set the various parameters:
</P><PRE
CLASS="PROGRAMLISTING"
>option        SYSVSHM
option        SHMMAXPGS=4096
option        SHMSEG=256

option        SYSVSEM
option        SEMMNI=256
option        SEMMNS=512
option        SEMMNU=256
option        SEMMAP=256</PRE
><P>
       </P
><P
>        You might also want to configure your kernel to lock shared
        memory into RAM and prevent it from being paged out to swap.
        This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
        setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>HP-UX</SPAN
></DT
><DD
><P
>        The default settings tend to suffice for normal installations.
        On <SPAN
CLASS="PRODUCTNAME"
>HP-UX</SPAN
> 10, the factory default for
        <TT
CLASS="VARNAME"
>SEMMNS</TT
> is 128, which might be too low for larger
        database sites.
       </P
><P
>        <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> parameters can be set in the <SPAN
CLASS="APPLICATION"
>System
        Administration Manager</SPAN
> (<ACRONYM
CLASS="ACRONYM"
>SAM</ACRONYM
>) under
        <SPAN
CLASS="GUIMENU"
>Kernel
        Configuration</SPAN
>-&gt;<SPAN
CLASS="GUIMENUITEM"
>Configurable Parameters</SPAN
>. Choose
        <SPAN
CLASS="GUIBUTTON"
>Create A New Kernel</SPAN
> when you're done.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
></DT
><DD
><P
>        The default maximum segment size is 32 MB, which is only adequate
        for very small <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
        installations.  The default maximum total size is 2097152
        pages.  A page is almost always 4096 bytes except in unusual
        kernel configurations with <SPAN
CLASS="QUOTE"
>"huge pages"</SPAN
>
        (use <TT
CLASS="LITERAL"
>getconf PAGE_SIZE</TT
> to verify).  That
        makes a default limit of 8 GB, which is often enough, but not
        always.
       </P
><P
>        The shared memory size settings can be changed via the
        <TT
CLASS="COMMAND"
>sysctl</TT
> interface.  For example, to allow 16 GB:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmmax=17179869184</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmall=4194304</KBD
></PRE
><P>
        In addition these settings can be preserved between reboots in
        the file <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.  Doing that is
        highly recommended.
       </P
><P
>        Ancient distributions might not have the <TT
CLASS="COMMAND"
>sysctl</TT
> program,
        but equivalent changes can be made by manipulating the
        <TT
CLASS="FILENAME"
>/proc</TT
> file system:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 17179869184 &gt;/proc/sys/kernel/shmmax</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 4194304 &gt;/proc/sys/kernel/shmall</KBD
></PRE
><P>
       </P
><P
>        The remaining defaults are quite generously sized, and usually
        do not require changes.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Mac OS X</SPAN
></DT
><DD
><P
>        The recommended method for configuring shared memory in OS X
        is to create a file named <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>,
        containing variable assignments such as:
</P><PRE
CLASS="PROGRAMLISTING"
>kern.sysv.shmmax=4194304
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
kern.sysv.shmall=1024</PRE
><P>
        Note that in some OS X versions,
        <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all five</I
></SPAN
> shared-memory parameters must be set in
        <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>, else the values will be ignored.
       </P
><P
>        Beware that recent releases of OS X ignore attempts to set
        <TT
CLASS="VARNAME"
>SHMMAX</TT
> to a value that isn't an exact multiple of 4096.
       </P
><P
>        <TT
CLASS="VARNAME"
>SHMALL</TT
> is measured in 4 kB pages on this platform.
       </P
><P
>        In older OS X versions, you will need to reboot to have changes in the
        shared memory parameters take effect.  As of 10.5 it is possible to
        change all but <TT
CLASS="VARNAME"
>SHMMNI</TT
> on the fly, using
        <SPAN
CLASS="APPLICATION"
>sysctl</SPAN
>.  But it's still best to set up your preferred
        values via <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>, so that the values will be
        kept across reboots.
       </P
><P
>        The file <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
> is only honored in OS X
        10.3.9 and later.  If you are running a previous 10.3.x release,
        you must edit the file <TT
CLASS="FILENAME"
>/etc/rc</TT
>
        and change the values in the following commands:
</P><PRE
CLASS="PROGRAMLISTING"
>sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall</PRE
><P>
        Note that
        <TT
CLASS="FILENAME"
>/etc/rc</TT
> is usually overwritten by OS X system updates,
        so you should expect to have to redo these edits after each update.
       </P
><P
>        In OS X 10.2 and earlier, instead edit these commands in the file
        <TT
CLASS="FILENAME"
>/System/Library/StartupItems/SystemTuning/SystemTuning</TT
>.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>SCO OpenServer</SPAN
></DT
><DD
><P
>        In the default configuration, only 512 kB of shared memory per
        segment is allowed. To increase the setting, first change to the
        directory <TT
CLASS="FILENAME"
>/etc/conf/cf.d</TT
>. To display the current value of
        <TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>./configure -y SHMMAX</PRE
><P>
        To set a new value for <TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>./configure SHMMAX=<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></PRE
><P>
        where <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> is the new value you want to use
        (in bytes). After setting <TT
CLASS="VARNAME"
>SHMMAX</TT
>, rebuild the kernel:
</P><PRE
CLASS="PROGRAMLISTING"
>./link_unix</PRE
><P>
        and reboot.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Solaris</SPAN
> 2.6 to 2.9 (Solaris
      6 to Solaris 9)</DT
><DD
><P
>        The default maximum size of a shared memory segment is too low for
        <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. The relevant settings can be changed in
        <TT
CLASS="FILENAME"
>/etc/system</TT
>, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256

set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32</PRE
><P>
        You need to reboot for the changes to take effect.  See also
        <A
HREF="http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html"
TARGET="_top"
>http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html</A
>
        for information on shared memory under older versions of Solaris.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Solaris</SPAN
> 2.10 (Solaris
      10)<BR><SPAN
CLASS="SYSTEMITEM"
>OpenSolaris</SPAN
></DT
><DD
><P
>        In Solaris 10 and OpenSolaris, the default shared memory and
        semaphore settings are good enough for most
        <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> applications.  Solaris now defaults
        to a <TT
CLASS="VARNAME"
>SHMMAX</TT
> of one-quarter of system <ACRONYM
CLASS="ACRONYM"
>RAM</ACRONYM
>.  If
        you need to increase this in order to set shared memory settings
        slightly higher, you should use a project setting associated
        with the <TT
CLASS="LITERAL"
>postgres</TT
> user.  For example, run the
        following as <TT
CLASS="LITERAL"
>root</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>projadd -c "PostgreSQL DB User" -K "project.max-shm-memory=(privileged,8GB,deny)" -U postgres -G postgres user.postgres</PRE
><P>
       </P
><P
>        This command adds the <TT
CLASS="LITERAL"
>user.postgres</TT
> project and
        raises the shared memory maximum for the <TT
CLASS="LITERAL"
>postgres</TT
>
        user to 8GB, and takes effect the next time that user logs
        in, or when you restart <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (not reload).
        The above assumes that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is run by
        the <TT
CLASS="LITERAL"
>postgres</TT
> user in the <TT
CLASS="LITERAL"
>postgres</TT
>
        group.  No server reboot is required.
       </P
><P
>        Other recommended kernel setting changes for database servers which will
        have a large number of connections are:
</P><PRE
CLASS="PROGRAMLISTING"
>project.max-shm-ids=(priv,32768,deny)
project.max-sem-ids=(priv,4096,deny)
project.max-msg-ids=(priv,4096,deny)</PRE
><P>
       </P
><P
>        Additionally, if you are running <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
        inside a zone, you may need to raise the zone resource usage
        limits as well.  See "Chapter2:  Projects and Tasks" in the
        <I
CLASS="CITETITLE"
>Solaris 10 System Administrator's Guide</I
> for more
        information on <TT
CLASS="LITERAL"
>projects</TT
> and <TT
CLASS="COMMAND"
>prctl</TT
>.
       </P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>UnixWare</SPAN
></DT
><DD
><P
>        On <SPAN
CLASS="PRODUCTNAME"
>UnixWare</SPAN
> 7, the maximum size for shared
        memory segments is only 512 kB in the default configuration.
        To display the current value of <TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idtune -g SHMMAX</PRE
><P>
        which displays the current, default, minimum, and maximum
        values. To set a new value for <TT
CLASS="VARNAME"
>SHMMAX</TT
>,
        run:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idtune SHMMAX <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></PRE
><P>
        where <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> is the new value you want to use
        (in bytes). After setting <TT
CLASS="VARNAME"
>SHMMAX</TT
>, rebuild the
        kernel:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idbuild -B</PRE
><P>
        and reboot.
       </P
></DD
></DL
></DIV
><DIV
CLASS="TABLE"
><A
NAME="SHARED-MEMORY-PARAMETERS"
></A
><P
><B
>Table 17-2. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> Shared Memory Usage</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Usage</TH
><TH
>Approximate shared memory bytes required (as of 8.3)</TH
></TR
></THEAD
><TBODY
><TR
><TD
>Connections</TD
><TD
>(1800 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
></TD
></TR
><TR
><TD
>Autovacuum workers</TD
><TD
>(1800 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
></TD
></TR
><TR
><TD
>Prepared transactions</TD
><TD
>(770 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-resource.html#GUC-MAX-PREPARED-TRANSACTIONS"
>max_prepared_transactions</A
></TD
></TR
><TR
><TD
>Shared disk buffers</TD
><TD
>(<A
HREF="runtime-config-preset.html#GUC-BLOCK-SIZE"
>block_size</A
> + 208) * <A
HREF="runtime-config-resource.html#GUC-SHARED-BUFFERS"
>shared_buffers</A
></TD
></TR
><TR
><TD
>WAL buffers</TD
><TD
>(<A
HREF="runtime-config-preset.html#GUC-WAL-BLOCK-SIZE"
>wal_block_size</A
> + 8) * <A
HREF="runtime-config-wal.html#GUC-WAL-BUFFERS"
>wal_buffers</A
></TD
></TR
><TR
><TD
>Fixed space requirements</TD
><TD
>770 kB</TD
></TR
></TBODY
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN25659"
>17.4.2. Resource Limits</A
></H2
><P
>    Unix-like operating systems enforce various kinds of resource limits
    that might interfere with the operation of your
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server. Of particular
    importance are limits on the number of processes per user, the
    number of open files per process, and the amount of memory available
    to each process. Each of these have a <SPAN
CLASS="QUOTE"
>"hard"</SPAN
> and a
    <SPAN
CLASS="QUOTE"
>"soft"</SPAN
> limit. The soft limit is what actually counts
    but it can be changed by the user up to the hard limit. The hard
    limit can only be changed by the root user. The system call
    <CODE
CLASS="FUNCTION"
>setrlimit</CODE
> is responsible for setting these
    parameters. The shell's built-in command <TT
CLASS="COMMAND"
>ulimit</TT
>
    (Bourne shells) or <TT
CLASS="COMMAND"
>limit</TT
> (<SPAN
CLASS="APPLICATION"
>csh</SPAN
>) is
    used to control the resource limits from the command line. On
    BSD-derived systems the file <TT
CLASS="FILENAME"
>/etc/login.conf</TT
>
    controls the various resource limits set during login. See the
    operating system documentation for details. The relevant
    parameters are <TT
CLASS="VARNAME"
>maxproc</TT
>,
    <TT
CLASS="VARNAME"
>openfiles</TT
>, and <TT
CLASS="VARNAME"
>datasize</TT
>. For
    example:
</P><PRE
CLASS="PROGRAMLISTING"
>default:\
...
        :datasize-cur=256M:\
        :maxproc-cur=256:\
        :openfiles-cur=256:\
...</PRE
><P>
    (<TT
CLASS="LITERAL"
>-cur</TT
> is the soft limit.  Append
    <TT
CLASS="LITERAL"
>-max</TT
> to set the hard limit.)
   </P
><P
>    Kernels can also have system-wide limits on some resources.
    <P
></P
></P><UL
><LI
><P
>      On <SPAN
CLASS="PRODUCTNAME"
>Linux</SPAN
>
      <TT
CLASS="FILENAME"
>/proc/sys/fs/file-max</TT
> determines the
      maximum number of open files that the kernel will support.  It can
      be changed by writing a different number into the file or by
      adding an assignment in <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
      The maximum limit of files per process is fixed at the time the
      kernel is compiled; see
      <TT
CLASS="FILENAME"
>/usr/src/linux/Documentation/proc.txt</TT
> for
      more information.
      </P
></LI
></UL
><P>
   </P
><P
>    The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server uses one process
    per connection so you should provide for at least as many processes
    as allowed connections, in addition to what you need for the rest
    of your system.  This is usually not a problem but if you run
    several servers on one machine things might get tight.
   </P
><P
>    The factory default limit on open files is often set to
    <SPAN
CLASS="QUOTE"
>"socially friendly"</SPAN
> values that allow many users to
    coexist on a machine without using an inappropriate fraction of
    the system resources.  If you run many servers on a machine this
    is perhaps what you want, but on dedicated servers you might want to
    raise this limit.
   </P
><P
>    On the other side of the coin, some systems allow individual
    processes to open large numbers of files; if more than a few
    processes do so then the system-wide limit can easily be exceeded.
    If you find this happening, and you do not want to alter the
    system-wide limit, you can set <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s <A
HREF="runtime-config-resource.html#GUC-MAX-FILES-PER-PROCESS"
>max_files_per_process</A
> configuration parameter to
    limit the consumption of open files.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LINUX-MEMORY-OVERCOMMIT"
>17.4.3. Linux Memory Overcommit</A
></H2
><P
>    In Linux 2.4 and later, the default virtual memory behavior is not
    optimal for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. Because of the
    way that the kernel implements memory overcommit, the kernel might
    terminate the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server (the
    master server process) if the memory demands of
    another process cause the system to run out of virtual memory.
   </P
><P
>    If this happens, you will see a kernel message that looks like
    this (consult your system documentation and configuration on where
    to look for such a message):
</P><PRE
CLASS="PROGRAMLISTING"
>Out of Memory: Killed process 12345 (postgres).</PRE
><P>
    This indicates that the <TT
CLASS="FILENAME"
>postgres</TT
> process
    has been terminated due to memory pressure.
    Although existing database connections will continue to function
    normally, no new connections will be accepted.  To recover,
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will need to be restarted.
   </P
><P
>    One way to avoid this problem is to run
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> on a machine where you can
    be sure that other processes will not run the machine out of
    memory.  If memory is tight, increasing the swap space of the
    operating system can help avoid the problem, because the
    out-of-memory (OOM) killer is invoked only when physical memory and
    swap space are exhausted.
   </P
><P
>    On Linux 2.6 and later, it is possible to modify the
    kernel's behavior so that it will not <SPAN
CLASS="QUOTE"
>"overcommit"</SPAN
> memory.
    Although this setting will not prevent the <A
HREF="http://lwn.net/Articles/104179/"
TARGET="_top"
>OOM killer</A
> from being invoked
    altogether, it will lower the chances significantly and will therefore
    lead to more robust system behavior.  This is done by selecting strict
    overcommit mode via <TT
CLASS="COMMAND"
>sysctl</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>sysctl -w vm.overcommit_memory=2</PRE
><P>
    or placing an equivalent entry in <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
    You might also wish to modify the related setting
    <TT
CLASS="VARNAME"
>vm.overcommit_ratio</TT
>.  For details see the kernel documentation
    file <TT
CLASS="FILENAME"
>Documentation/vm/overcommit-accounting</TT
>.
   </P
><P
>    Another approach, which can be used with or without altering
    <TT
CLASS="VARNAME"
>vm.overcommit_memory</TT
>, is to set the process-specific
    <TT
CLASS="VARNAME"
>oom_adj</TT
> value for the postmaster process to <TT
CLASS="LITERAL"
>-17</TT
>,
    thereby guaranteeing it will not be targeted by the OOM killer.  The
    simplest way to do this is to execute
</P><PRE
CLASS="PROGRAMLISTING"
>echo -17 &#62; /proc/self/oom_adj</PRE
><P>
    in the postmaster's startup script just before invoking the postmaster.
    Note that this action must be done as root, or it will have no effect;
    so a root-owned startup script is the easiest place to do it.  If you
    do this, you may also wish to build <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
    with <TT
CLASS="LITERAL"
>-DLINUX_OOM_ADJ=0</TT
> added to <TT
CLASS="VARNAME"
>CPPFLAGS</TT
>.
    That will cause postmaster child processes to run with the normal
    <TT
CLASS="VARNAME"
>oom_adj</TT
> value of zero, so that the OOM killer can still
    target them at need.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    Some vendors' Linux 2.4 kernels are reported to have early versions
    of the 2.6 overcommit <TT
CLASS="COMMAND"
>sysctl</TT
> parameter.  However, setting
    <TT
CLASS="LITERAL"
>vm.overcommit_memory</TT
> to 2
    on a 2.4 kernel that does not have the relevant code will make
    things worse, not better.  It is recommended that you inspect
    the actual kernel source code (see the function
    <CODE
CLASS="FUNCTION"
>vm_enough_memory</CODE
> in the file <TT
CLASS="FILENAME"
>mm/mmap.c</TT
>)
    to verify what is supported in your kernel before you try this in a 2.4
    installation.  The presence of the <TT
CLASS="FILENAME"
>overcommit-accounting</TT
>
    documentation file should <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> be taken as evidence that the
    feature is there.  If in any doubt, consult a kernel expert or your
    kernel vendor.
   </P
></BLOCKQUOTE
></DIV
></DIV
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